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Re: [PATCH] Unify pthread_once (bug 15215)
- From: Rich Felker <dalias at aerifal dot cx>
- To: OndÅej BÃlka <neleai at seznam dot cz>
- Cc: Carlos O'Donell <carlos at redhat dot com>, Torvald Riegel <triegel at redhat dot com>, GLIBC Devel <libc-alpha at sourceware dot org>, libc-ports <libc-ports at sourceware dot org>
- Date: Mon, 26 Aug 2013 22:29:12 -0400
- Subject: Re: [PATCH] Unify pthread_once (bug 15215)
- Authentication-results: sourceware.org; auth=none
- References: <1368024237 dot 7774 dot 794 dot camel at triegel dot csb> <519D97E4 dot 4030808 at redhat dot com> <20130826124955 dot GA6065 at domone dot kolej dot mff dot cuni dot cz> <20130826164507 dot GA20515 at brightrain dot aerifal dot cx> <20130826184150 dot GA8772 at domone dot kolej dot mff dot cuni dot cz>
On Mon, Aug 26, 2013 at 08:41:50PM +0200, OndÅej BÃlka wrote:
> > No, pthread_once _calls_ tend to be once per access to an interface
> > that requires static data to have been initialized, so possibly very
> > often. On the other hand, pthread_once only invokes the init function
> > once per program instance. I don't see anything that would typically
> > happen once per thread, although I suppose you could optimize out
> > calls to pthread_once with tls:
> >
> Could happen often but dees it? Given need of doing locking you need to
> avoid it in performance critical code. With once per thread I meant an
> patterns:
>
> computation(){
> pthread_once(baz,init); // Do common initialization.
> pthread_create(foo,bar,routine);
> }
>
> or
>
> pthread_create(foo,bar,routine);
>
> with
>
> routine()
> {
> pthread_once(baz,init); // Do common initialization.
> ...
> }
These patterns arise is the library is making threads and using
pthread_once to initialize its static data before making the thread.
I'm thinking instead of the case where your library is being _called_
by multi-threaded code, and using pthread_once to ensure that its data
is safely initialized even if there are multiple threads which might
be racing to be the first caller.
> > static __thread int once_done = 0;
> > static pthread_once_t once;
> > if (!once_done) {
> > pthread_once(&once, init);
> > once_done = 1;
> > }
> >
> > This requires work at the application level, though, and whether it's
> > a net advantage depends a lot on whether multiple threads are likely
> > to be hammering pthread_once on the same once object, and whether the
> > arch has expensive acquire barriers and inexpensive TLS access.
> >
> Actually you can use following if you are concerned about that use cases:
>
> #define pthread_once2(x,y) ({ \
> static __thread int once = 0; \
> if (!once) \
> pthread_once(x,y); \
> once=1; \
> })
Indeed; actually, this could even be done in pthread.h, with some
slight variations, perhaps:
#define pthread_once(x,y) ({ \
pthread_once_t *__x = (x); \
static __thread pthread_once_t *__once; \
if (__once != __x) { \
pthread_once(__x,y); \
__once = __x; \
} \
})
Rich